Does soil composition have an influence on the sandblasting process ?




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Abstract

Introduction
Raman lidars operating at 355 and 532-nm wavelengths are very useful for the vertical profiling of dust optical properties. In contrast to standard lidars they allow an unambiguous determination of the volume extinction coefficient of dust particles (Ansmann et al., 1990). In contrast to insitu observations (aircraft) they allow a profiling under ambient conditions and over long time periods. In contrast to satellite and ground-based remote sensing vertically resolved information on dust layering is obtained so that a clear separation between the optical effects caused by particles in the boundary layer (mixture of dust, marine and anthropogenic particles) and in the free troposphere (mainly mineral particles) is possible. It is essential to know the amount of dust in the free troposphere. This fraction can be transported over thousands of kilometers and thus can affect climate on a hemispheric scale. Profiling of the extinction coefficient at 355 and 532 nm is useful because Saharan dust seems to considerably absorb radiation at 355 nm. At 532 nm, light absorption does not play an important role.
Furthermore, simultaneous profiling of the extinction coefficient and the backscatter coefficient (180 degree scattering) with Raman lidar provides us with vertically resolved values of the extinction-to-backscatter ratio (lidar ratio, Ansmann et al., 1992). This quantity is very interesting for dust studies because it is sensitive to particle shape. Backscattering by nonspherical dust particles is believed to be a factor of 2-3 (Mishchenko et al., 1997) or even 3-10 (Kalashnikova et al., 2002) lower than backscattering by surface-equivalent spheres. Our observations support this assumption (Mattis et al., 2002; Müller et al., 2003). The lidar ratio is also an input parameter in the estimation of dust extinction profiles with standard backscatter lidars. Thus, observed (not simulated) dust lidar ratios are needed for an improved dust monitoring with standard lidars In near future satellite-based standard lidars may map the global aerosol distribution in terms of backscattering. A global aerosol-type-dependent lidar ratio climatology is then needed to obtain reliable 3-D particle extinction information. It should be mentioned that the number of aerosol Raman lidars is growing. About 50 % of the stations of the European Aerosol Research Lidar Network (EARLINET, Bösenberg et al., 2001) is equipped with Raman channels. Many Saharan dust outbreaks were observed with EARLINET between 2000 and 2002.

Observations
Here, we report on two major outbreaks that took place in August and October 2001. We will present results in terms of measured spectrally resolved backscatter and extinction profiles, lidar-ratio and depolarization-ratio profiles, and dust optical depth. The observations were made at Leipzig, Germany, which is a site of the Aerosol Robotic Network (AERONET) as well as of EARLINET (Müller et al., 2003). During the two outbreaks, about 80% of the dust was found in the free troposphere. The dust layers were mainly below 6 km height, with traces of dust up to 10 km height. The particle depolarization ratio ranged from 10%-25%. The African air masses travelled 3000-5000 km before arriving at Leipzig, Germany. Optical depths were as high as 0.3 to 0.6 at 532 nm. Extinction coefficients ranged from 100 to 300 Mm-1 at both wavelengths. The Angstrom exponent (355/532 nm) varied from 0-0.5. Single scattering values were close to 0.95 (derived from AERONET Sun photometer observations). The effective radius of the dust particles, estimated from the combined lidar/photometer data, was about 0.5 µm. This value is much smaller than typical literature values. Unexpectedly large lidar ratios, mainly between 50 and 80 sr at 532 nm, were observed in the free-tropospheric dust layers. The values for 355 nm were, on average, 10%-30% larger. Based on Mie scattering calculations dust lidar ratios were expected to be close to 40 sr (355 nm) and 20 sr (532 nm). The higher lidar ratio for 355 nm (for spherical particles) results from larger absorption in the UV. However, a clear interpretation of the wavelength dependence of the observed dust lidar ratios is not possible because of the dependence of the lidar ratio on the size distribution, chemical composition, and especially on particle shape. In contrast to the extinction coefficient (and its spectral slope), particle backscattering and its wavelength dependence is believed to be strongly influenced by shape effects (Mishchenko et al., 1997).

Current problems
Multiwavelength Raman lidars (backscatter at 355, 532, 1064nm, extinction at 355, 532nm) can in principle be used to retrieve microphysical properties (surface and volume concentrations of the particles, refractive index) from the optical data (Müller et al., 2001). Because of the sensitive impact of particle shape on the backscatter coefficients such inversions (exclusively based on lidar data) are not possible in the case of desert dust. Combined photometer/Raman lidar observations may improve the situation. The latest status in this field is given in the workshop poster presented by Müller et al.. More work (model calculations, closure studies with lidar, radiometers, aircraft) is needed to provide the lidar community with a clear picture of the dependence of the backscatter coefficient (at the widely used wavelengths of 355, 532, and 1064 nm) on realistic dust particle shapes and particle distributions for a mixture of different shapes. Raman lidar observations in southern Italy (much closer to the source region, eastern part of the Sahara; dust over Leipzig originated mainly from the western part of the Sahara) yield much lower lidar ratios (50-60 sr) at 355 nm (Leipzig: 60-90 sr), much closer to the values found from Mie calculations (De Tomasi et al., 2003). After long-range transport of Asian dust lidar ratios over Japan ranged mostly from 40-55 sr at 532 nm (Liu et al., 2002). These values are lower than the lidar ratios observed over Leipzig (mostly 60-75 sr), but clearly larger than the results from Mie calculations. So, presently there is large room for speculations about the impact of the shape of the dust particles on the measurable optical properties. It should also be examined in which way the depolarization ratio measured with lidar depends on the size distribution and the shape of the particles. How useful are depolarization ratio observations at two wavelengths (e.g., 532 and 1064 nm)? Could such observations be used to retrieve the mean size of the particles or to identify the most probable shape of the particles?

References

  • Ansmann, A., M. Riebesell, and C. Weitkamp, 1990: Measurements of atmospheric aerosol extinction profiles with Raman lidar, Opt. Lett., 15, 746-748.

  • Ansmann, A., U. Wandinger, M. Riebesell, C. Weitkamp, and W. Michaelis, 1992: Independent measurement of extinction and backscatter profiles in cirrus clouds by using a combined Raman elastic-backscatter lidar, Appl. Opt., 31, 7113-7131.

  • Bösenberg, J., et al., 2001: EARLINET: A European Aerosol Research Lidar Network, in Laser remote sensing of the atmosphere. Selected papers of the 20th International Laser Radar Conference, Vichy, France, A. Dabas, C. Loth, and J. Pelon, Eds., Ecole Polytechnique, Paris, France, 155-158.

  • De Tomasi, F., A. Blanco, and M. R. Perrone, 2003: Raman lidar monitoring of extinction and backscattering of African dust layers and dust characterization, Appl. Opt., 42, 1699-1709.

  • Kalashnikova, O. V., and I. N. Sokolik, 2002: Importance of shapes and compositions of wind-blown dust particles for remote sensing at solar wavelengths, Geophys. Res. Lett., 29, 10.1029/2002GL014947.

  • Liu, Z., N. Sugimoto, and T. Murayama, 2002: Extinction-to-backscatter ratio of Asian dust observed with high-spectral-resolution-lidar and Raman lidar, Appl. Opt., 41, 2760-2767.

  • Mattis I., A. Ansmann, D. Müller, U. Wandinger, and D. Althausen, 2002: Dual-wavelength Raman lidar observations of the extinction-to-backscatter ratio of Saharan dust, Geophys. Res. Lett., 29, 9, 10.1029/2002GL014721.

  • Mishchenko, M. I., L. D. Travis, R. A. Kahn, and R. A. West, 1997: Modeling phase functions for dustlike tropospheric aerosols using a shape mixture of randomly oriented polydisperse spheroids, J. Geophys. Res., 102, 16831-16847.

  • Müller, D., U. Wandinger, D. Althausen, M. Fiebig, 2001: Comprehensive particle characterization from three-wavelength Raman-lidar observations: case study, Appl. Opt., 40, 4863-4869.

  • Müller D., I. Mattis, U. Wandiger, D. Althausen, A. Ansmann, O. Dubovik, S. Eckhardt, and A. Stohl, 2003: Saharan dust over a central European EARLINET-AERONET site: combined observations with Raman lidar and Sun photometer, J. Geophys. Res., 108, 10.1029/2002JD002918.

 

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The Dirt on Dust

Richard Arimoto

Carlsbad Environmental Monitoring & Research Center, New Mexico State University, Carlsbad, NM, USA arimoto@cemrc.org




Abstract

The composition and concentrations of atmospheric dust have been characterized in various ways, perhaps most commonly by determining the concentrations of dust-associated elements. Typically, Al or Fe or Si or some other element is used as an indicator of dust, often with the tacit assumption that all other sources for that indicator are negligible; in some cases water soluble Ca has been used for this purpose. Elemental ratios are then calculated and compared with a compositional reference to determine which elements are in crustal proportions and which are not. There are several problems with the reference element approach for evaluating dust concentrations and composition, the first being weathering reactions which can cause coatings, such as rock varnishes, to form on the parent materials from which the dust is formed. Second, alluvial fans and playas are important sources of dust in some places, and as a result pedogenic salts can be mixed with crustal material when the eolian dust forms. These two effects cause differences between the composition of the eolian dust relative to the crustal reference, but they would likely be minor compared with the next problem, which is that non-dust sources can emit aerosols that in some ways resemble mineral dust; that is, the "sole source" assumption for the indicator element is violated. For instance, the concentrations of some major elements such as Al, Fe and Ca in coal fly ash are similar to those of typical soils, but various other elements are strongly enriched in the coal combustion residues.
Recent studies conducted as part of ACE-Asia showed that at Zhenbeitai, People's Republic of China, which is close to the Asian dust source region, the molar ratio of sulfate to soluble calcium was ~0.1, but the ratio increased to ~1 at Gosan, South Korea, which is much farther from the main Asian dust sources. One explanation for these results is that gas-to-particle conversion caused the observed increase in sulfate relative to Ca downwind, that is, SO2 gas or H2SO4 vapor became associated with the dust particles during transit. However, both the percentage of Ca that was soluble and the ratio of soluble calcium to aluminum were lower at Gosan than Zhenbeitai, indicating that aerosol mixing also affected the Ca concentrations during transport. Yet another process that could have contributed to the observed differences between sites is the size-selective fractionation of dust during transport; this process can compromise the indicator element approach by preferentially removing minerals that do not have the indicator and other elements in crustal proportions.
Even natural substances such as atmospheric sea salt can cause complications with the indicator element approach if the amount of the interfering substance is sufficiently high. At Bermuda, a small island in the North Atlantic, sea-salt Al can amount to as much as ~30% of the total monthly aerosol Al, even though such high percentages occur for only a few months each year when the dust concentrations and deposition rates were low. While the presence of non-dust materials is probably inconsequential in many cases, interferences of this nature can call into question inferences about dust concentrations based on indicator elements. The essence of the problem in using indicator elements is that no elements are unique to dust.
Studies recently conducted in Carlsbad, NM, USA suggest that mineral dust is contaminated with transuranic radionuclides from atmospheric nuclear weapons tests-the question raised by these studies is whether the relationship between dust and bomb-derived nuclides is restricted geographically or a widespread phenomenon, perhaps even involving global contamination. Related studies by other investigators have shown that dust concentrations can be correlated with those of organic nitrogen, and the possibility of associations between microbes, pesticides and persistent organochlorine compounds with dust also has been raised. An important implication of the widespread contamination of dust--beyond artifacts in determining dust concentrations--is that the optical and radiative properties of contaminated dust likely differ significantly from those of pure mineral aerosol. Without information on the extent and effects of contamination, the characterization of pure dusts' composition and properties may prove to be more of academic exercise than a key to improving assessments of aerosols' impact on climate.

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HETEROGENEOUS CHEMISTRY ON MINERAL AEROSOL: INFLUENCE ON TROPOSPHERIC OZONE

Y. Balkanski, M. Schulz, S. Textor, D. Hauglustaine, S.E. Bauer, R. van Dingenen 2, P. Bonasoni 3, H. Fischer 4 F. Dentener 2, J. P. Putaud 2

IPSL-LSCE, l'Orme des Merisiers, Bât 709, 91191 Gif/Yvette Cedex France
Joint Research Center, I-21020, Ispra, Italy
  CNR ISAC, via Gobetti, 40129, Bologna, Italy 
Max Planck Institute for Chemistry, POB 3060, 55020 Mainz, Germany




Abstract

The progresses made in understanding the role of mineral aerosol in chemistry and climate have been very important in the last few years. Satellite, sunphotometers and Lidars have allowed to document the large regions prone to the influence of these aerosols. Very pointed techniques used in the laboratory and in the field have evidenced that the surface of the aerosol allow for heterogeneous reactions to take place in particular for HNO3, HO2, H2O2, SO2 which in turn affects NOx, NOy and O3.
The effect on ozone of heterogeneous reactions at the surface of mineral aerosol was investigated using a General Circulation Model coupled to a module INCA that treats INteractions between Chemistry and Aerosols. The accomodation coefficients of H2SO4, NO3, O3 and N2O5 were chosen according to laboratory experiments that were conducted within the framework of the European project MINATROC (MINeral Aerosol and Tropospheric Chemistry). Two field experiments one that took place in Mt Cimone, Italy in June 2000 and the second one in Izana in July-August 2002 provide a characterisation of the gas and aerosols phase at the sites that is compared to model results.
The results of a nudged global simulation for the years 2000 and 2002 permit to estimate the effect of the heterogeneous reactions of mineral dust. Tropospheric ozone is reduced by 10 to 30% in the tropics and the relative roles of H2SO4, NO3, O3 and N2O5 in this reduction have been sorted out. We will also point to significant reduction in H2SO4 and O3 predicted over Southern Europe and Asia.

References

  • Balkanski, Y., S. E. Bauer, R. van Dingenen, P. Bonasoni, M. Schulz, H. Fischer, G. P. Gobbi, M. Hanke, D. Hauglustaine, J. P. Putaud, A. Stohl, and F. Raes , The Mt Cimone, Italy, free tropospheric campaign: principal characteristics of the gaseous and aerosol composition from European pollution, Mediterranean influences and during African dust events,  Atmos. Chem. Phys. Discuss., 3, 1753-1776, 2003.

  • Bauer S. E., Y. Balkanski, M. Schulz, D. Hauglustaine and F. Dentener, Heterogeneous chemistry on mineral dust aerosol surfaces: Influence on the global tropospheric ozone chemistry, to be submitted to J. Geophys. Res., 2003.

  • Guelle W., Y. Balkanski, M. Schulz, B. Marticorena, G. Bergametti, C. Moulin, R. Arimoto, And K. D. Perry, Modelling the atmospheric distribution of mineral aerosol: Comparison with ground measurements and satellite observations for yearly and synoptic time scales over the North Atlantic, J. Geophys. Res, 105, 1997-2005, 2000.

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Satellite views of spatial, seasonal and interannual variability of AFRICAN dust and relationships with meteorology and climate: combination of TOMS and METEOSAT observations

I. Chiapello (1), C. Moulin (2), S. Benaissa (1) and M. Legrand (1)

(1) Laboratoire d'Optique Atmosphérique, Université des Sciences et Technologies de Lille, France, chiapello@loa.univ-lille1.fr, fax :+33-3-20-43-43-42
(2) Laboratoire des Sciences du Climat et de l'Environnement, CE Saclay, Gif-sur-Yvette, France




Abstract

In the recent years, several studies have investigated the mineral dust variability at interannual time scales on the basis of individual satellite records, either over Africa using METEOSAT/infrared observations [Brooks and Legrand, 2000], or over surrounding oceanic regions using METEOSAT/VIS and TOMS (Total Ozone mapping Spectrometer) observations [Moulin et al., 1997; Chiapello and Moulin, 2002]. These studies have highlighted the potential impact of several climate parameters on the dust export, especially the North Atlantic Oscillation (NAO) and the Sahel drought, but are generally limited in time (~10 years) and regionally. To better understand these impacts, it is necessary to enlarge at maximum the period for which satellite observations are available and to combine the different satellite records available.
The daily TOMS/Nimbus-7 (1979-1992) and TOMS/Earth Probe (1997-2000) Aerosol Index (AI) have been combined with the METEOSAT/VIS (1984-1997) dust optical thickness (DOT) over the Atlantic ocean to derive an estimated TOMS DOT since 1979 over both ocean and land. These estimated TOMS DOT are validated over Africa by comparison to Sun-Photometer measurements performed during field campaigns in the 1980's and derived from the AERONET network. The temporal and spatial variability of African dust derived from these estimates are analyzed and compared to those derived by METEOSAT/VIS over the Atlantic and by METEOSAT/Infrared over Sahara and Sahel. For these three data sets, inter-annual variations of dust loads are investigated in conjunction with annual variations of rainfall in Sahel and North Atlantic Oscillation.

References

  • Brooks, N. & Legrand, M. Dust variability over northern Africa and rainfall in the Sahel, S.J. McLaren and D.R. Kniveton (eds.), Linking Climate Change to Land Surface Change, Kluwer Academic Publishers, 1-25 (2000).

  • Chiapello, I. & Moulin, C. TOMS and METEOSAT satellite records of the variability of Saharan dust transport over the Atlantic during the last two decades (1979-1997). Geophys. Res. Lett. 29, 17-20 (2002).

  • Moulin, C. et al. Control of atmospheric export of dust from North Africa by the North Atlantic Oscillation. Nature 387, 691-694 (1997).

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Retrieval of Optical Properties of Desert Dust Aerosol from AERONET Observations

Oleg Dubovik1,2, Brent Holben1, Tom F. Eck1,2, Alexander Smirnov1,2, Tatyana Lapyonok1,3, Alexander Sinyuk1,2 , Didier Tanre4 Philippe Goloub4 and Ilya Slutsker1,3

1 NASA Goddard Space Flight Center, Greenbelt, MD, USA
2 Goddard Earth Science and Technology Center, University of Maryland Baltimore County, USA
3 Science Systems and Applications Inc., Lanham, USA
4 Universite de Science et Techniques de Lille, Lille, France
dubovik@aeronet.gsfc.nasa.gov, brent@aeronet.gsfc.nasa.gov, teck@aeronet.gsfc.nasa.gov, asmirnov@aeronet.gsfc.nasa.gov, sinyuk@aeronet.gsfc.nasa.gov, lapyonok@aeronet.gsfc.nasa.gov, Didier.Tanre@univ-lille1.fr, goloub@loaser.univ-lille1.fr , ilya@aeronet.gsfc.nasa.gov,




Abstract

This presentation outlines the main features of desert dust optical properties observed in the AERONET retrievals.
INTRODUCTION
AERONET - AErosol RObotic NETwork (Holben et al. 1998) of ~ 150 identical globally distributed sun and sky scanning ground-based automated radiometers provides measurements of desert dust optical properties in many locations. The spectral sky-radiance is measured in a wide angular range from the sun and is minimally affected by surface reflectance. The standardized network procedures (Holben et al. 1998, Smirnov et al. 2000) of instrument maintenance, calibration, cloud screening and data processing allow for quantitative comparison of the aerosol data obtained at different times and geographical sites.
The inversion algorithm (Dubovik and King, 2000) employed by AERONET provides aerosol retrievals by fitting the entire measured field of radiances - optical depths and the angular distribution of sky radiances - at four wavelengths (0.44, 0.67, 0.87 and 1.02 µm) to a radiative transfer model. The radiation field is driven by the (wavelength dependent) aerosol complex index of refraction and the particle size distribution (22 size bins in the range: 0.05 ≤ r ≤ 15 µm) in the total atmospheric column. Only spectral and size smoothness constraints are used, preventing unrealistic oscillations in either parameter.
RESULTS
Size distributions of desert dust are typically bimodal and dominated by coarse mode (super micron radius) particles. Correspondingly, the Angstrom parameter is low (ranges from ~ 0.75 down to - 0.1) and the phase function asymmetry is relatively high at all wavelengths considered. Some differences for dust of different geographic origin are also observed. For example, the desert dust from the western part of Africa and the Saudi Arabian Peninsula (Saudi Arabia and Cape Verde) are strongly dominated by large particles (Cvc/Cvf ~ 50) and seem to have optical properties more representative of so-called pure desert dust. The aerosol in Bahrain/Persian Gulf has a larger fine mode (Cvc/Cvf ~ 10) than observed in Saudi Arabia and in Cape Verde. This difference relates to the frequent presence in the Persian Gulf of small particles produced by industrial activity. The median sizes range from ~0.12 to ~0.15 µm for fine mode and from ~1.9 to ~2.6 µm for coarse modes and, in contrast with biomass burning and urban/industrial aerosols do not show any pronounced dynamics with aerosol loading.
Single Scattering albedo ω0 values retrieved by AERONET for Saharan dust (0.96 - 0.99 for wavelengths greater than 550 nm) are significantly higher than many aerosol models suggest. Similarly, the retrieved imaginary part of the refractive index k(λ) ranging from 0.0006 to 0.003 are smaller than the 0.008 value given for the visible spectrum by several models. Another feature of the retrievals is the pronounced absorption of desert dust the blue spectral range (ω0(440) ~ 0.92 - 0.93 and k(λ) is 3-4 times higher at 440 than at the longer wavelengths). Such spectral dependence is not surprising for desert dust and has been reported previously in many studies (however for higher absolute values of absorption). Asian dust in spring, measured over urbanized China, exhibited greater absorption (ω0 ~ 0.92 - 0.94 for wavelengths > 550 nm) due to mixing with absorbing fine mode particles and probable attachment of BC to the dust surfaces. The typical values of real part of the refractive index retrieved by AERONET ranging from 1.48 to 1.56 for various dust observation are in general agreement with most available dust measurements.
Particle nonsphericity was consistently observed in desert dust retrievals as the appearance of retrieval artifacts (high concentration of very small particles with r < 0.1 µm and strong spectral dependence of n(λ)) associated with the presence of nonspherical particles (as shown in sensitivity studies by Dubovik et al, 2000). The similarity of these retrieval features with ones observed in numerical tests suggested that a nonspherical scattering model of randomly oriented spheroids is rather adequate for desert dust aerosol. Therefore, we have developed an approach allowing (Dubovik et al, 2002) the use of the model of dustlike aerosol particles as polydisperse, randomly oriented spheroids in retrieving aerosol optical properties from remote measurements of atmospheric radiances. The application to the entire AERONET database has shown significant improvements in the retrieved size distribution, refractive index, and phase function for aerosols in desert dust-dominated or influenced locations.

References

  • Dubovik, O., A. Smirnov, B. N. Holben, M. D. King, Y. J. Kaufman, T. F. Eck and I. Slutsker, 2000: Accuracy assessment of aerosol optical properties retrieval from AERONET sun and sky radiance measurements. J. Geophys. Res., 105, 9791-9806.

  • Dubovik, O., and M. D. King, 2000: A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements. J. Geophys. Res., 105, 20673-20696.

  • Dubovik, O., B. N. Holben, T. Lapyonok, A. Sinyuk, M. I. Mishchenko, P. Yang and I. Slutsker, Non-spherical aerosol retrieval method employing light scattering by spheroids, Geophys. Res. Lett., 10.1029/2001GL014506, 2002.

  • Holben, B. N., T. F. Eck, I. Slutsker, D. Tanré, J. P. Buis, A. Setzer, E. Vermote, J. A. Reagan, Y. J. Kaufman, T. Nakajima, F. Lavenu, I. Jankowiak and A. Smirnov, 1998: AERONET-A federated instrument network and data archive for aerosol characterization. Remote Sens. Environ., 66, 1-16.

  • Smirnov A., B. N. Holben, T. F. Eck, O. Dubovik and I. Slutsker, 2000: Cloud screening and quality control algorithms for the AERONET data base. Remote Sens. Environ., 73, 73,337-73349.

 

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About Mineral Dust Deposition

François Dulac

Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Unité Mixte de Recherche CNRS-CEA No. 1572, CEA Saclay 709, F-91191 Gif-Sur-Yvette Cedex, France
e-mail fdulac@cea.fr




Abstract

This presentation aims at pointing out the need for experimental studies of mineral dust deposition and at raising some relevant points and questions.
The study of mineral dust deposition is of interest because Aeolian erosion in arid and semi-arid areas and subsequent long-range transport of mineral dust particles are responsible for huge transfers of matter from continent to continent and from continent to oceans, which affect soil and sediment formation and biogeochemical cycles.
First, it is of interest to keep in mind that dust concentration in ice or sediment cores is primarily a record of deposition processes. Present observations show that atmospheric dust load and deposition at a given place may be uncorrelated and exhibit different seasonal patterns.
A number of dust deposition studies indicate that the annual fallout is often dominated by a few events. Although wet deposition generally dominates yearly budgets, dry deposition may be dominant during half-year long dry seasons so that both should be assessed separately. There are also large interannual variations in mineral dust deposition. Strategies for measuring mineral dust deposition should therefore rely on a continuous monitoring basis over multi-year periods, with a high temporal resolution of the order of one or few days. The use of automated rain collectors makes it possible, and should be recommended for monitoring wet deposition on an event basis. Immediate filtration to apportion chemicals between the soluble and particulate phases should also be encouraged.
Dust deposition is, indeed, known to be a source of limiting nutrients (e.g. Si, Fe, P, ...) for surface waters of the open ocean. Model studies suggest that high dust deposition rates to the world ocean at the Last Glacial Maximum may have impacted atmospheric CO2 concentrations by 30-50 ppm through Fe fertilization. However, we have a very limited understanding of the extent to which dust deposition provides bio-available nutrients. First laboratory studies suggest that there are probably differences in the bio-available fraction from deposited dust between wet and dry deposition, and this may be of interest to study biogeochemical impacts. It can also be questioned whether monthly averaged deposition fluxes are relevant to study biogeochemical impact on surface ocean, due to the sporadic nature of deposition events.
Deposition data have proved much helpful for validating models of aerosol transport. There has been, however, few attempts to validate mineral dust deposition fields, and only integrated deposition budgets are generally considered. In order to promote detailed model validation, a tentative inventory of existing data on mineral dust deposition will be made, and technical aspects of measurements discussed.
Finally, some data on particle size distribution from deposition samples will be considered. Deposition sampling appears like a good tool to study the largest particle size fraction of mineral dust which is likely underestimated by classical measurements made on aerosol particles or on sampled air.

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Passive Visible and Infrared Observations of Dust Aerosol Properties

Philip A. Durkee

Professor of Meteorology Naval Postgraduate School Monterey, California USA




Abstract

The effects of dust have been observed in satellite imagery since the early visible wavelength radiometers. Today, on the order of two dozen satellites orbit the earth with the ability to sense dust and its properties. Detecting the presence of dust is relatively easy compared to many other aerosol types. However, quantifying the radiative properties of dust and its impact on radiative heating, visibility, and general human activity is quite difficult. This talk will describe some of those difficulties, illustrate the consequences of various assumptions and approximations, and describe some techniques for limiting the ambiguity inherent in the problem through the use of multiple satellite/sensor analysis.
The workhorse for climatological aerosol observations, beginning in the 1970's, has been the Advanced Very High Resolution Radiometer (AVHRR) on board the NOAA series of satellites. AVHRR currently measures radiance in 5 wavelength bands including three reflected solar bands (0.63, 0.86, and 1.6 ?m band center wavelengths). With three bands, analysis of AVHRR radiance measurements can provide aerosol optical depth estimates by constraining the possible combinations of particle size and absorption characteristics that are consistent with the radiance measurements. The problem of course has many more than three degrees of freedom, so the results are dependent on the a priori assumptions about particle size and absorption properties of the aerosol.
Recent multispectral radiometers such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) or the MOderate Resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua satellites display beautiful renditions of dust events by exploiting the subtle differences in the reflectance of airborne dust compared to clouds or land surfaces. These sensors, along with the new MEdium Resolution Imaging Specrometer (MERIS) on ENVISAT, and the GLobal Imager (GLI) on the Midori-II satellite, provide an unprecedented multispectral view of the reflected solar radiance from the Earth's aerosol systems. A summary of the sensor characteristics is provided below.
Solar spectral radiance measurements do not tell the whole story. Radiance measurements as a function of scattering angle and polarization also provide important information to constrain estimates of aerosol properties. The Multiangle Imaging SpecrtroRadiometer (MISR) on the Terra satellite measures radiance at 4 wavelengths from 9 view angles and the Advanced Along Track Scanning Radiometer (AATSR) on ENVISAT measures radiance at 7 wavelengths from two directions. The POLarization and Directionality of the Earth's Reflectances (POLDER) instrument on Midori-II measures polarization parameters.
In addition, since dust aerosol is of sufficient size and concentration, infrared radiance also carries information about aerosol properties. MODIS, AVHRR, GOES, and the new Meteosat Second Generation (MSG) sensors all measure radiance in multiple infrared channels that provide information about dust aerosol.
This talk will illustrate the various dust observation techniques using combinations of passive visible and infrared sensors. The focus will be on a summary of what is possible with these techniques and not a comprehensive review due to time constraints and some techniques will be more adequately described by other speakers. The limitations of these techniques will also be discussed. Finally, future developments and improvements in dust aerosol characterization from passive visible and infrared measurements will be described.
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